Recent advances in wireless communication technology have allowed an entire RF transceiver to be implemented on a single semiconductor die or chip. However, integrating a complete RF transceiver on a single chip presents a number of challenges. For example, in wideband code division multiple access (WCDMA) transceivers, a single-chip solution requires two RF VCOs to be running on the chip at the same time. Such an arrangement may produce undesired interaction between the two VCOs due to various types of mutual coupling mechanisms, which may result in spurious receiver responses and unwanted frequencies in the transmit spectrum. The primary mutual coupling mechanism is usually the fundamental electromagnetic (EM) coupling between the resonators, i.e., the large inductor structures in the VCOs.
A number of techniques exist for reducing the mutual EM coupling between the VCOs due to the inductors. One technique involves reduction of EM coupling by careful design of the inductors to provide maximum isolation of the inductors. Another techniques calls for frequency separation by operating the two VCOs at different even harmonics of the desired frequency. Still another technique involves frequency separation by using a regenerative VCO concept. The frequency separation methods exploit the filtering properties of the resonator to reduce interference. However, these solutions require additional circuitry (dividers, mixers, etc.) that may increase current consumption, making them less attractive than other mutual EM coupling reduction alternatives.